Energy Balances in Stirred Tank Reactors: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • The reactors discussed in this module are mostly, but not exclusively, used for liquid phase reactions.
  • The tanks are assumed to be gradientless for the reactors in this module. This means the concentrations and temperatures are spatially uniform throughout the reactor. This also means that if the reactor has an effluent stream, its concentrations and temperature are identical to the corresponding values in the reactor.
  • The energy balances in this module assume kinetic and potential energy changes are negligible, which is true for most chemical reactors.
  • The work term is often small for tank reactors, but stirring can add significant energy if the reactor contents are viscous.
  • Phase changes are not included in the energy balances in this module, but they can be important for some reactors. For example, a liquid-phase reaction may form a vapor product.
  • Feed entering a stirred tank reactor is assumed to be quickly mixed (relative to the residence time in the reactor), so that the feed undergoes a step change in temperature.
  • The energy balance and the mass balances are solved simultaneously, and are usually solved numerically.
  • An energy balance for an unsteady-state tank reactor can be applied to:
    • Start-up of a CSTR
    • A semibatch reactor (the inlet or outlet flow rates may be zero)
    • A batch reactor (inlet and outlet flow rates are zero)
    • Steady-state CSTR (the derivative with respect to time is zero) 
From studying this module, you should now be able to:
  • Write energy balances for batch reactors, semibatch reactors, and unsteady-state continuous stirred tank reactors (CSTRs).
  • Solve mass balances and an energy balance simultaneously for a tank reactor that is not at steady-state.
  • Solve mass and energy balances for a steady-state CSTR.


Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder